A TIG (Tungsten Inert Gas) welding method in which an arc is generated between a non-consumable tungsten electrode and a workpiece in an inert gas atmosphere has been widely used because a high-quality welded part is obtained. However, as compared to other welding methods such as a MAG (Metal Active Gas) welding method or a MIG (Metal Inert Gas) welding method, the TIG welding method has problems in that the welding speed is slow and the welding efficiency is decreased.
On the other hand, the MAG welding method and the MIG welding methods are welding methods in which an arc is generated between a consumable welding wire electrode and a workpiece in an active gas atmosphere and in an inert gas atmosphere, respectively. These methods have a higher welding efficiency than that of the TIG welding method as described above but have a problem in that spatter is likely to occur. Further, the MAG welding method has a problem in that the toughness of weld metal is likely to be decreased.
Here, the reason why spatter is likely to occur in the MAG welding method and the MIG welding method is that short-circuiting is likely to occur between a distal end of a welding wire electrode and a base metal. In addition, the reason why the toughness is likely to be decreased in the MAG welding method is that oxidized gas in a shielding gas is melted in weld metal and the amount of oxygen in the weld metal is increased.
In order to compensate for the problems of the methods, various TIG-MIG hybrid welding methods have been proposed (for example, PATENT DOCUMENT 1).
When the MIG welding method is carried out on carbon steel or stainless steel using inert gas such as argon or helium as a shielding gas, a cathode spot is not fixed and an arc is unstable.
In contrast, in the TIG-MIG hybrid welding method, metal vapor is generated by the leading TIG arc, and electric current flow is formed in the metal vapor. In such a case, a molten pool generated by the TIG arc has a lower work function than solid metal and easily discharges electrons. Therefore, the cathode spot of the MIG arc is easily fixed to the molten pool.
Therefore, with the TIG-MIG hybrid welding method, carbon steel or stainless steel can be stably welded in inert gas used as a shielding gas, and the amount of oxygen dissolved in the weld metal can be decreased. In addition, in the TIG-MIG hybrid welding method, when a wire of a MIG welding electrode is about to come into contact with base metal, a distal end of the wire is melted and separated as a droplet by the heating action of a TIG arc. Therefore, short-circuiting does not occur between the wire and the base metal and spatter can be prevented.
As described above, the TIG-MIG hybrid welding method can compensate for the problems of TIG welding and MAG (or MIG) welding but has a unique problem due to its property of arc rigidity. Here, the arc rigidity refers to the property in which, even when an electrode is inclined, an arc is likely to be generated straight in an extending direction of a tungsten electrode or a wire. In the TIG-MIG hybrid welding method, since a TIG arc and a MIG arc, which have current flowing directions opposite each other, are generated close to each other, arc repulsion is generated by an electromagnetic force.
As a result, the TIG-MIG hybrid welding method of the related art has a problem in that an arc is likely to be unstable due to arc rigidity and arc repulsion which work in different directions. When the arc is unstable, there is a problem in that a weld bead is irregular and a blow hole is likely to be formed.
Incidentally, in order to decrease the arc repulsion in the TIG-MIG hybrid welding method of the related art, a hot-wire TIG welding method has been used (for example, PATENT DOCUMENT 2).
FIG. 12 illustrates a conventional hot-wire TIG welding method. As illustrated in FIG. 12, in the hot-wire TIG welding method, an arc is not generated from a wire, and welding is carried out using resistance heating caused by allowing a current to flow through the wire. As a result, a MIG arc disappears and arc repulsion does not occur. Therefore, arc stability can be improved.
Therefore, in a wire heating power source of the hot-wire TIG welding method, the voltage is controlled to be low (for example, 6 V to 7 V), unlike in a MIG welding power source of the TIG-MIG hybrid welding method in which the voltage is controlled to be high (for example 13 V to 30 V) in order to generate an arc between a wire and base metal.    PATENT DOCUMENT 1: Japanese Unexamined Patent Application, First Publication No. S53-34653    PATENT DOCUMENT 2: Japanese Unexamined Patent Application, First Publication No. H6-79466